All children attending Head Start preschools across the country would have to be vaccinated under a bill two California members of Congress plan to introduce next week.

The legislation by Democrats U.S. Sen. Barbara Boxer and Rep. Anna Eshoo would only allow parents to opt out of vaccinating their kids if a doctor signs off saying a medical condition prevents it, such as a child undergoing chemotherapy. Parents who currently have unvaccinated children in Head Start and Early Head Start programs would have three months to get their shots up to date, according to a statement announcing the bill.

“More than a million of our children attend Head Start programs all over the country, and we must protect every single one of these kids from preventable diseases like measles,” Boxer said in a statement. “This simple bill is an important step toward strengthening our vaccination policies at all levels of government to prevent the spread of deadly diseases.”

The proposal comes as measles – long thought to be eradicated in the U.S. – spreads and California lawmakers consider state-level legislation to tighten vaccination requirements for school kids. Under a bill state Sen. Richard Pan, D-Sacramento plans to introduce in the state Legislature, parents would no longer be allowed to opt out of immunizing their kids by stating they have a personal belief against vaccines.

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Head Start is a federally-funded program that provides preschool to children from low-income families. Early Head Start is a similar program for pregnant women, infants and toddlers. A recent analysis by the Los Angeles Times found that Head Start programs had among the highest measles vaccination rates of all preschools in California, at 96 percent. That compares with a vaccination rate of about 87 percent of toddlers and preschoolers in private child care facilities, the Times reported.